WOOD STOCK IN NEOTROPICAL STREAMS: QUANTIFYING AND COMPARING INSTREAM WOOD AMONG BIOMES AND REGIONS

In-stream wood plays important chemical, physical and ecological functions in aquatic systems, benefiting the biota directly and indirectly. However, human activities along river corridors have disrupted wood recruitment and retention leading to reduced in-stream wood amounts. In the tropics, where wood is assumed to be more transient, the expansion of agriculture and infrastructure might be reducing in-stream wood stock even more than in the better studied temperate streams. However, research is needed to augment the small amount of information about wood in different biomes and ecosystems of neotropical streams. Here we present the first extensive in-stream wood assessment in wet-tropical Amazon and semi-humid-tropical Cerrado (the Brazilian savanna) catchments, describing the in-stream wood loads and size distributions. We also compare neotropical wood stocks with those from temperate streams, first contrasting with literature data and then with a comparable dataset from USA temperate biomes. Contrary to our expectations Amazon and Cerrado streams had similar wood loads, which are lower than the world literature average, but similar to those found in comparable temperate forest and savanna streams in USA. Our results indicate that the field survey methods and the wood metric examined are highly important when comparing different datasets. But when properly compared, we found that most of the wood in temperate streams is made-up of a small number of large pieces, whereas wood in neotropical streams is made up of larger number of small pieces that produce similar total volumes. The character of wood volumes among biomes is linked more to the delivery, transport and decomposition mechanisms than to the total number of pieces. Future studies should further investigate the potential in-stream wood drivers in neotropical catchments in order to better understand the differences and similarities here detected between biomes and bioclimatic regions. 

Impact/Purpose

In-stream wood derived from riparian trees is among the most important contributors to instream habitat complexity, retention of nutrients, and stabilization of banks and bed sediments. The depletion and removal of wood from streams from deforestation and stream channel management is recognized as a global issue threatening the biodiversity of streams world-wide. Neotropical streams are hotspots of biodiversity, but many are also located in areas of rapid human development. Deforestation and degradation of riparian zones, aggravated by channelization and loss of connectivity in streams and rivers, interrupt wood supply and storage, resulting in lower wood loads than would be observed under least-disturbed reference conditions. Unfortunately, there is a great gap in knowledge about the amount and size of wood in these streams. In this study, the authors conduct the first-ever large-scale examination of the amount of wood in streams within tropical rainforest (Amazon) and savanna (Cerrado) regions of Brazil, using field and analytical methods developed by EPA’s National Aquatic Resource Surveys (NARS). They compare the amounts of wood in these streams with those in many biomes around the world and discuss the limitations resulting from differences in methods used to measure wood among published studies. Because they used the same field methods as NARS, however they were able to do a rigorous comparison of instream wood in neotropical ecoregions with those in the nine major ecoregions of the USA. Contrary to their expectations Amazon and Cerrado streams had similar wood loads, which are lower than the world literature average, but similar to those found in comparable temperate forest and savanna streams in USA. Their results indicate that the field survey methods and the choice of wood metric examined are highly important when comparing different datasets. They found that even when wood contained in riparian vegetation is similar, the amount and sizes of instream wood among biomes reflects differences in mechanisms of delivery, transport and decomposition.

Citation

Saraiva, S., I. Rutherfurd, P. Kaufmann, C. Leal, D. Macedo, AND P. Pompeu. WOOD STOCK IN NEOTROPICAL STREAMS: QUANTIFYING AND COMPARING INSTREAM WOOD AMONG BIOMES AND REGIONS. Public Library of Science, San Francisco, CA0275464, (2022). [DOI: 10.1371/journal.pone.0275464]

DOI: WOOD STOCK IN NEOTROPICAL STREAMS: QUANTIFYING AND COMPARING INSTREAM WOOD AMONG BIOMES AND REGIONS